A new approach to electrical heating appliances in recent years has been self-regulating heating systems which utilize materials exhibiting certain types of PTC (positive temperature coefficient) of resistance characteristics. The distinguishing characteristic of the prior art PTC materials is that upon attaining a certain temperature, a substantial rise in resistance occurs. Prior art heaters utilizing PTC materials reportedly exhibit more or less sharp rises in resistance within a narrow temperature range, but below that temperature range exhibit only relatively small changes in resistance with temperature. The temperature at which the resistance commences to increase sharply is often designated the switching or anomaly temperature (T.sub.s) since on reaching that temperature the heater exhibits an anomalous change in resistance and for practical purposes, switches off. Self regulating heaters utilizing PTC materials have advantages over conventional heating apparatus in that they generally eliminate the need for separate thermostats, fuses or in-line electrical resistors.
The most widely used PTC material has been doped barium titanate which has been utilized for self-regulating ceramic heaters employed in such applications for food warming trays and other small portable heating appliances. Although such ceramic PTC materials are in common use for heating applications, their rigidity has severely limited the number of applications for which they can be used. PTC materials comprising electrically conductive polymeric compositions are also known and certain types have been shown to possess the special characteristics described herein-above. However, in the past, use of such polymeric PTC materials has been relatively limited, primarily due to their low heating capacity. Such materials generally comprise one or more conductive fillers such as carbon black or powdered metal dispersed in a crystalline thermoplastic polymer. PTC compositions prepared from highly crystalline polymers generally exhibit a steep rise in resistance commencing a few degrees below their crystalline melting point similar to the behavior of their ceramic counterparts at the Curie temperature (the T.sub.s for ceramics). PTC compositions derived from polymers and copolymers of lower crystallinity, for example, less than about 50%, exhibit somewhat less steep increases in resistance which increase commences at a less well defined temperature in a range often considerably below the polymer's crystalline melting point. In the extreme case some polymers of low crystallinity yield resistance vs temperature curves which are more or less convex upwards. Other types of thermoplastic polymers yield resistances which increase fairly smoothly and more or less steeply but continuously with temperature. FIG. I illustrates characteristic curves for the aforementioned different types of PTC compositions. In FIG. I curve 1 exhibits the sharp virtually right angle increase in resistance (hereinafter known as type I behavior) generally characteristic of (inter alia) polymers having high crystallinity; curve 2 shows the more gradual increase at lower temperatures (relative to the polymer melting point) hereinafter known as type II behavior generally characteristic of lower crystallinity polymers. Curve 3 (Type III behavior) illustrates the convex upward curve characteristic of many very low crystallinity polymers while curve 4 (Type IV behavior) illustrates the large increase in resistance without a region of more or less constant resistance (at least in the temperature range of commercial interest) seen with some materials. Curve 5 (Type V behavior) illustrates the gently increasing resistance temperature characteristic shown by many prior art electrical resistors. Although the above types of behavior have been illustrated mostly by reference to specific types of polymeric material it will be realized by those skilled in the art that the particular type of behavior manifested is also very dependent on the type and amount of conductive filler and, particularly in the case of carbon black, on its particle size and shape, surface characteristics, tendency to agglomerate and the shape of the particle agglomerates (i.e., its tendency to structure).
It should be noted that the preferred PTC compositions taught by the prior art are all indicated to manifest essentially Type I behavior. In fact, the prior art does not specifically recognize the Types II to V behavior notwithstanding the fact that many of the PTC compositions taught by the prior art in fact exhibit not Type I, but rather Type II, III, IV, or V behavior.
With type I resistance temperature characteristics, the increase in resistance above T.sub.s is rapid so that T.sub.s may be regarded as the temperature at which the device switches off. However, with type II or type III PTC materials the transition from a resistance relatively stable as temperature is increased to a resistance rising steeply with temperature is much less well defined. Thus, the anomaly temperature or T.sub.s is frequently not an exact temperature. Thus, though we will describe a device as shutting off at a precise temperature T.sub.s, it will be understood by those skilled in the art that in many practical instances it may be appropriate to understand T.sub.s as being the lowest temperature of a range in temperature over which the device switches off or, indeed, to consider T.sub.s to be a relatively narrow temperature range rather than a discrete temperature.
Prior art disclosed self-regulating thermal devices utilizing a PTC material contemplate extremely steep (Type I) R=f (T) curves so that above a certain temperature the device will in effect shut off, while below that temperature a relatively constant wattage output at constant voltage is achieved. At temperatures below T.sub.s the resistance is at a relatively low and constant level and thus the current flow is relatively high for any given applied voltage (I=E/R). The power generated by this current flow is dissipated as Joule heat, i.e. heat generated by electrical resistance=I.sup.2 R, (E.sup.2 /R) thereby warming up the PTC material. The resistance stays at this relatively low level until about the T.sub.s temperature, at which point a rapid increase in resistance occurs. With the increase in resistance there is a concomitant decrease in power, thereby limiting the amount of heat generated so that when the T.sub.s temperature is reached heating it essentially stopped. Then, upon a lowering of the temperature of the device below the T.sub.s temperature by dissipation of heat to the surroundings, the resistance drops, thereby increasing the power output.
At a steady state, the heat generated will essentially balance the heat dissipated. Thus, when an applied voltage is directed across a PTC heating element, the Joule heat causes heating of the PTC element up to about its T.sub.s, (the rapidity of such heating depending on the applied voltage and type of PTC element), after which little additional temperature rise will occur due to the increase in resistance. Because of the resistance rise, a PTC heating element will ordinarily reach a steady state at approximately T.sub.s thereby self-regulating the heat output of the element without resort to fuses or thermostats. The advantages of such a self contained heat regulating element in many applications should be apparent, in that the need for expensive and/or bulky heat control devices such as thermostats is eliminated.
Kohler, U.S. Pat. No. 3,243,753 discloses carbon filled polyethylene wherein the conductive carbon particles are in substantial contact with one another. Kohler contemplates a product containing 40% polyethylene and 60% carbon particles so as to give a resistance at room temperature of about 1 ohm/inch. As is typical of the alleged performance of the prior art materials, Kohler's PTC product is characterized by a relatively flat curve of electrical resistance versus temperature below the switching temperature, followed by a sharp rise in resistivity of at least 250% over a 25.degree. F. range. The mechanism suggested by Kohler for the sharp rise in resistivity is that such change is a function of the difference in thermal expansion of the materials, i.e. polyethylene and particulate carbon. It is suggested that the composition's high level (i.e. 60%) of conductive filler forms a conductive network through the polyethylene polymer matrix, thereby giving an initial constant resistivity at lower temperatures. However, at about its crystalline melt point, the polyethylene matrix rapidly expands, such expansion causing a breakup of many of the conductive networks, which in turn results in a sharp increase in the resistance of the composition.
Other theories proposed to account for the PTC phenomenon in conductive particle filled polymer compositions include complex mechanisms based upon electron tunnelling through inter grain gaps between particles of conductive filler or some mechanism based upon a phase change from crystalline to amorphous regions in the polymer matrix. A background discussion of a number of proposed alternative mechanisms for the PTC phenomenon is found in "Glass Transition Temperatures as a Guide to the Selection of Polymers Suitable for PTC Materials", J. Meyer, Polymer Engineering and Science, November, 1973, Volume 13, No. 6. This same J. Meyer in U.S. Pat. No. 3,673,121 suggests that, based upon a phase change theory, to attain a steeply sloped PTC of resistance with a sharp cutoff (Type I) the polymer matrix should comprise a crystalline polymer having a narrow molecular weight distribution. Kawashima et al, U.S. Pat. No. 3,591,526 discloses a PTC molding composition in which the conductive particles, such as carbon black, are first dispersed in a thermoplastic material, and thereafter this dispersed mixture is blended into a molding resin. Kawashima et al likewise suggests the desirability of an extremely steep temperature-resistance curve (that is, R=f (T)) curve at a T.sub.s of about 100.degree.-130.degree. C.
Because of their flexibility, comparatively low cost, and ease of installation, PTC strip heaters comprising conductive particles dispersed in a crystalline polymer have recently found wide use as pipe tracing heaters on industrial piping and in related applications. For example, such polymeric PTC heaters, because of their self-regulating features, have been used for wrapping pipes in chemical plants to protect against freezing, or for maintaining a constant temperature which in turn permits aqueous or other solutions to flow through the pipes without "salting out".
In such applications, heaters ideally attain and are maintained at a temperature at which the energy lost through heat transfer to the surroundings equals that gained from the current. Such heaters ordinarily consist of a relatively narrow and thin ribbon or strip of carbon filled polymeric material having electrodes (such as embedded copper wires) at opposite edges along the long axis of the strip. Thus an electrical potential gradient along the plane of and transverse to the long axis of the strip has generally been contemplated, an applied voltage between the opposite electrodes resulting in heating of the entire strip, usually to approximately its T.sub.s.
Obviously, from the preceeding discussion it is apparent that Type I materials have significant advantages over the other types of PTC material enumerated hereinbefore in most applications. Types II and III have a disadvantage in that because of the much less sharp transition the steady state temperature of the heater is more dependant on the thermal load placed on it. Such compositions also suffer from a current inrush problem as described in greater detail hereinafter. Type IV and V PTC materials, because they lack a temperature range in which the power output is not essentially independent of temperature have not so far been considered as suitable materials for practical heaters under ordinary circumstances.
In such uses as have been described above and in others there exists a need for flexible strip heaters with much higher power output densities and/or higher operating temperatures than are contemplated by the prior art. We have found that attempts to operate heaters, particularly strip heaters fabricated from prior art compositions and according to prior art designs at higher power outputs, i.e., higher wattage levels (above 1.5 watts/sq. in.) and/or higher temperatures, (above about 100.degree. C.) fail. The actual wattage delivered by prior art heaters is far less than that which would be expected based on the heater area and heat transfer considerations apparently because the heat is produced in a very thin band down the long axis of the strip between the two electrodes. Such a phenomenon, which is unrecognized by the prior art, we term "hotline". This hotline phenomenon results in an inadequate and nonuniform heating performance and renders the entire heating device useless for most of the heating cycle in applications where high wattage outputs, especially at temperatures above 100.degree. C., are desired. More specifically, because the heat output is confined to a narrow band or line transverse to a current path, the high resistance of this line prevents the flow of current across the path, in effect causing the entire heater to shut off until the temperature of the hotline drops to the T.sub.s temperature range again.
Indeed, in certain instances the heater may be permanently damaged in the hotline area.
We have discovered that this hotline condition occurs in most if not all prior art design polymeric PTC strip heaters where a voltage is applied, i.e. the current flows transversely across the strip, the extent of such condition being generally dependent upon the amount of applied voltage as well as the thermal conductivity of the polymer and the extent of non-uniform dissipation. The hot-line along the long axis of the strip, between the side electrodes, effectively shuts down the heating device even though only a small portion of the surface area of the film, i.e. the hot line, has achieved the T.sub.s temperature. This, in many cases, will destroy the heater or at the very least render it so inefficient that it appears to exhibit the very low heating capability we find to be generally associated with the PTC polymeric strip heaters of the prior art.
From the foregoing discussion, it is apparent that the elimination of hotline is critical for the efficient operation of a PTC self-regulating heater, especially one with a high power output and/or high operating temperature.
It would also be most advantageous if a PTC self-regulating heater could be fabricated wherein the heating surface was of a shape other than a relatively long, narrow strip e.g., a square or round heating pad. Also desirable would be a PTC self regulating heater which could be fabricated into relatively complex three-dimensional configurations, e.g., essentially the entire outside surface of a chemical process vessel. Unfortunately, the tendency to hot-line is particularly prevalent where the current path distance i.e. the distance between electrodes, is large relative to the crosssectional area per unit length of PTC material through which the current must flow. For example, in the case of a heating strip with electrodes at the strip edges, a relatively wide short strip has a greater tendency to hot-line than a narrow strip of the same length, composition and thickness. Likewise, for the same length and width, the thinner the strip the greater the tendency to hot-line. Increasing strip length with width and thickness held constant has no significant effect on hot-lining tendency. None of the prior art workers have even recognized the problem of hot-lining, must less suggested a heater composition and/or construction which ameliorates the problem.
Polymeric PTC compositions have also been suggested for heat shrinkable articles. For example, Day in U.S. Patent Office Defensive Publication No. T905,001 teaches the use of a PTC heat shrinkable plastic film. However, the Day shrinkable film suffers from the rather serious shortcoming that since T.sub.s is no greater than the crystalline melting point of the film, very little recovery force can be generated. Buiting et al, U.S. Pat. No. 3,413,442 suggests a heater construction involving sandwiching a polymeric layer between silver electrodes. A significant shortcoming of the Buiting et al construction is its lack of flexibility. Additionally, neither Buiting et al nor any of the other previously discussed prior art teachings even addresses, much less solves, certain additional problems inherent in all prior art PTC heaters. First, is the problem of current inrush. This problem is particularly severe when it is desired to provide a heater having a T.sub.s in excess of about 100.degree. C. Many applications could advantageously utilize self-regulating heaters having a T.sub.s of 200.degree. C. or even more. Unfortunately, as heretofore indicated, known prior art PTC heater constructions are essentially unsuitable for such high T.sub.s applications.
With materials having a T.sub.s substantially above 100.degree. C., the resistance of such material at or just below the T.sub.s temperature may be as much as 10 times its resistance at ambient temperature. Since the PTC heater ordinarily functions at or slightly below its T.sub.s, its effective heat output is determined by its resistance at slightly below T.sub.s. Therefore, a PTC heater drawing, for example, 15 amps at 200.degree. C. could easily draw 150 amps at ambient temperature. Such a heater system would require a current carrying capacity vastly in excess of that required for steady state operation or, alternatively, require the installation of complex and generally fragile or expensive control circuitry to prevent the 150 amp initial current inrush from burning out the heater or lead wires thereto when the heater is first connected to an electrical source.
Referring to FIG. 2, the preferred type of heater characteristic (line ABC) in its ideal form has a constant resistance (denoted by the line AB) up to T.sub.s and a resistance which increases extremely rapidly (denoted by the line BC) above the T.sub.s. Thus, the operating range, say from its maximum rate to .about.0 current drawn, is as shown by the dotted lines intersecting the resistance temperature curve at B and D. The power output of the ideal heater is unaffected by changes in temperature below T.sub.s but changes over its whole range in a very small range of temperatures above T.sub.s. Unfortunately, as hereinbefore described, very few, if any, PTC materials actually display this ideal type of characteristic. The nearest one can usually get with practical heaters is shown by the lines AB' and B'C'. If the maximum permissible power drawn from the electrical circuit is given by the resistance at A, then the operating range for self limiting or "controlling" is given by the portion of the line B'C' lying between the dotted lines. Obviously, the heater temperature, when operating under "controlling" conditions, varies much more in this latter instance and the available power range in the "controlled" region is less than that in the ideal case. If a power range equal to that of the ideal case is desired, then a resistance characteristic such as A'B"C" is necessary.
Referring again to FIG. 2, a curve AEF represents a portion of the resistance characteristic of a PTC material of type II. If, as in the previous instance, the operating power range is set by the dotted resistance lines, it can be readily appreciated that the temperature of the heater will vary over quite wide limits in operation depending on the thermal load.
Although, as hereinabove mentioned, the prior art recognizes the considerable advantage of having a heater composition which possesses a resistance temperature characteristic of Type I, many of the compositions alluded to in the prior art show behavior more closely resembling Type II, or even Type III behavior. The optimum (Type I) characteristic is shown only by a limited selection of compositions and there has been a long felt need for a means of modifying compositions showing Type II or III behavior so that the behavior becomes or at least closely approaches that of Type I.
An additional problem inherent in all prior art PTC strip heaters is that when it is desired to heat an irregularly shaped substrate the heater must be wrapped around the substrate, generally resulting in certain portions of the strip fully or partially overlapping other portions. This overlap can cause irregular heating.
It is thus apparent that while a variety of PTC compositions and constructions are well-known to the prior art, all such compositions and constructions and indeed, any apparent combination thereof, possess serious shortcomings which severely limit the use of self-regulating PTC heating articles.
It is therefore one object of this invention to provide a design for a self-regulating heating article which ameliorates the problems of hot-lining, high currenct inrush and burn out but which is nevertheless suitable for high operating temperatures and/or current densities.
It is a further object of this invention to provide a self-regulating heating article construction which is readily fabricated into a variety of shapes including strip form or complex configurations and which exhibits good heat output even at comparatively low input voltages.
It is a further object of this invention to provide heaters manifesting behavior approximating the desirable Type I resistance-temperature characteristic from Type II, III or IV PTC materials.
It is a further object of this invention to provide heaters approximating a Type I temperature-resistance characteristic in which the power output of the heater below the T.sub.s of the PTC material is substantially unaffected by changes in temperature.
It is a further object of this invention to provide heaters whose T.sub.s temperatures are substantially independent of the nature of the PTC polymeric constituent.
It is a further object of this invention to provide a self-regulating heating article which possesses sufficient flexibility to be used in conjunction with a heat activated adhesive and/or a heat recoverable tape, sheet, tube or the like member which envelopes a substrate whereby said article's heat output effects activation of the adhesive and/or recovery of the heat recoverable member to thereby seal and/or encapsulate the substrate.
These and other objects and advantages of the instant invention will be further discussed or made apparent in conjunction with the detailed description thereof and of the embodiments, examples, and illustrations thereof set forth hereinafter.